This invention concerns improvements in and relating to signal transmission devices, more especially, but not exclusively, voice frequency transmission devices such as line repeaters, impedance matching devices, or hybrid amplifiers for incorporation in telephone communication systems.
In order to achieve amplification of the bidirectional signals occurring in a telephone line, a so-called hybrid circuit is conventionally used, whereby the single bidirectional speech channel provided by the telephone line is converted into two separate channels, in each of which speech transmission is effected in only one direction, the respective directions of the two channels being opposite to one another. Such an arrangement simplifies the processing of the telephone speech signals, since each channel can be dealt with separately and amplified or attenuated as desired. However, the provision of a hybrid circuit which provides adequate separation of the two directional speech channels whilst at the same time providing a path which is transparent to d.c. current signals such as are required to be transmitted over the telephone line, has hitherto presented considerable difficulties.
Various forms of hybrid circuit have hitherto been proposed, including the hybrid transformer, and various forms of transformerless or electronic hybrid circuits. All such circuits, however, require that the telephone line be terminated at respective sides of the voice frequency transmission path provided by the hybrid circuit, whilst a bypass path which is opaque to voice frequency signals is provided for transmission of the direct current signals. Moreover, the ports to the voice frequency transmission paths provided by the hybrid circuit must have input and output impedances matching those of the telephone line, whilst the bypass path presents a low impedance to direct current signals but effectively blocks voice frequency signals. In hitherto known circuit arrangements it has not been possible to achieve this object without at the same time introducing across the d.c. current path a capacitance which can interfere with the signalling function of d.c. signals on the telephone line. In particular, the signalling capacitance of the hybrid circuit may be such as to prevent remote testing of the capacitance of the telephone line itself, whereby the appropriate test can only be achieved by temporarily removing the hybrid circuit from the telephone line. This constitutes a serious disadvantage of known hybrid circuit arrangements.